1. Field of the Invention
The present invention relates to a control robot which has a machining tool comprising a rotatable tool for grinding at the distal end of the robot arm thereof so as to carry out grinding work with the machining tool pressing against the surface of a work to be machined under a predetermined pressure, and particularly relates to a force control robot for controlling a robot by detecting pressing force of the machining tool against a work to be machined.
2. Description of the Prior Art
As an example of conventional control robots, there is a robot which has a machining tool, such as a grinder, through a spring or damper at the distal end of the robot arm thereof, and presses the machining tool against a work under predetermined pressure by means of the urging force of the spring or damper.
However, in such a robot having so-called compliance by means of a spring or damper, though it is possible to weaken the impact force applied to the robot, or to machine the work with force limited in some range when a great control force is generated on contact of the tool against the work, it is difficult to control the pressing force, so that it is impossible to machine works with high accuracy. Moreover, such a robot is likely to be ill-balanced by the weight of the machining tool provided at the robot arm thereof, so that it is difficult to control the pressing force at a constant value.
As another example of conventional control robots, there is a robot which has one or two shafts for controlling the pressing direction at the robot arm.
However, in such a robot, since the one or two control shafts are added anew, the arm portion becomes large in size, so that the weight of the portion is greatly increased.
Moreover, as still another example, there are various studies on a control robot which has a six-axes force/ torque sensor for detecting the pressing force at the robot arm so as to control the respective shafts to adjust the force at a predetermined value. However, in such a case, since the respective shafts are driven so as to press the machining tool under predetermined pressure in a predetermined direction, the coordinate transformation becomes complex and a great amount of calculation is required. Moreover, there is also required troublesome calculation for the weight compensation for the six-axes force/torque sensor provided for the machining tool, which is changed with postures of the robot. Accordingly, the trouble for such computer operation is increased, moreover, an extremely high-speed computer is required. Besides, in this case, because the inertia force of each shaft changes greatly with postures of the robot and the rigidity of the robot main body changes according to use conditions, it is difficult to control the pressing force with high accuracy. Therefore, such a robot can not be applied to various machining conditions and working postures.
As stated above, in the conventional control robots, it is difficult to carry out the machining work with high accuracy. Moreover, to control the pressing force more precisely, it is necessary to add control shafts for the force control anew and an extremely great amount of calculation is required therefor. Besides, the machining condition and working posture are limited to small ranges.
Moreover, in a force control robot having a six-axes force/torque sensor between a machining tool, for example a grinder, and the robot arm so as to press the tool against a work with a predetermined force in an optional direction, the force along each shaft and moment about each shaft, or the synthesized force of these, each detected by the six-axes force/torque sensor is so controlled as to be a predetermined value.
Namely, compliance control or hybrid control is carried out by directly detecting the direction of force or moment and incorporating data on the detected force or moment in a control loop.
However, in such a robot, because the weight of the machining tool attached at the distal end of the torque sensor is relatively large, when the tool is moved at high acceleration, the inertia force generated by the acceleration is detected by the torque sensor.
Moreover, by such a detection method of the torque sensor, it is impossible to discriminate between the pressing force and the inertia force.
Accordingly, in such a construction of the above-mentioned control robot, it is difficult to measure only the pressing force applied to a work from the machining tool.
In such a conventional force detection method, even when the tool is not in contact with a work, a data on the inertia force generated by movement of the tool and vibration of the arm is transferred to the control system without discrimination from the pressing force. Moreover, since the inertia force is as large as or larger than the pressing force, it can not be ignored. If the inertia force is ignored on the machining work, it is impossible to carry out the work desirably.
Even though the vibration of the tool is very weak, when the inertia force generated thereby is transferred to the control system, the vibration is likely to be increased.
Accordingly, it is very difficult to increase the gain of the force control loop. Moreover, it is impossible to carry out control operation with good response, and it is also difficult to realize high accuracy machining work.
Moreover, a great amount of calculation is always required for the operation of a six-axes force/torque sensor, which depends on the posture of the machining tool.
Accordingly, troublesome computer operation is increased.
As described above, in such a conventional control robot, because of generation of the inertia force of the machining tool provided at the distal end of the robot arm, it is difficult to correctly detect the pressing force applied to a work from the tool, moreover, it is impossible to increase the gain of the force control loop. Therefore, the force control can not be carried out with good response, and it is difficult to realize high accuracy machining work.
Moreover, it is also necessary to calculate the weight compensation required for the six-axes force/torque sensor.
While, in the conventional control robot, when the shape of a work to be machined is known, it is possible to carry out machining work by pressing the tool along a normal of the work based on the shape and by always setting the machining tool in a predetermined posture corresponding to the work by changing the posture. However, when the shape of the work is not known in advance, since the robot has no function to judge which posture is correct, it can not be applied to such a case.
Moreover, even when the work shape is already known, the work required to teach the robot the shape or to input data corresponding to the shape to the control system is more trouble as the shape becomes more complex.
While, though now still being studied, there is a proposition about a robot in which a grindstone in a special form and a special force sensor are incorporated in the machining tool so as to grind a work of unknown shape. This robot can not be applied to wide use, so that it is difficult to use this robot for general grinding works. Moreover, since it is necessary to incorporate the grindstone of a special form and the special force sensor in the machining tool, a high production cost is required.
The conventional force control robot or force control apparatus can not be applied to a work of unknown shape to be machined. Even if possible, an extremely large amount of trouble is required for teaching the robot the shape of the work or inputting the data.